ARDMORE, Pa. (AP) -- Phil Mickelson made his first birdie on his last putt. Billy Horschel never missed a green. It was all they could do to barely break par against Merion, which is turning out to be the real star of this U.S. Open.

Nearly half the field did not finish the second round when it was suspended by darkness. Moments after the horn sounded to stop play, Mickelson opted to finish his round and drilled a 20-foot birdie putt for a 2-over 72. That gave him a share of the clubhouse lead with Horschel, who made it as easy as possible by hitting every green in regulation for a 67.

They were at 1-under 139.

Even with the round not finished, it was becoming clear that this U.S. Open might be up for grabs until the very end. Tiger Woods, who grimaced with every shot out of the rough because of pain in his left elbow, was at 3-over 143 and still very much in the game.

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"I don't know how anyone is going to separate too far from the field," Mickelson said. "There might be a hot round tomorrow, and they might get a hot round on Sunday, but unlikely to be the same player."

No one was hotter than Horschel, playing in his first U.S. Open since he was a 19-year-old in college.

Nothing is tougher than Merion, the little course in the tony suburbs of Philadelphia that even in rain-softened conditions is showing plenty of might. And to think there was chatter at the start of the week about the potential for the first 62 in major championship history.

"Perhaps next time you guys will believe when we say it's really not that easy, that it's really not that easy," Geoff Ogilvy said after a 70. That put him at 4-over 144, which gave him and dozens of others a legitimate shot going into the weekend.

The surprise were a pair of amateurs -- Michael Kim of Cal and Cheng-Tsung Pan of Taiwan. They were 2 under for their round and among those who didn't finish.

The long day, brought on by storm delays on Thursday, began with cool conditions and patches of light rain that eventually gave way to sunshine. That led players to wonder how much tougher Merion will be once it starts to dry out.

"It's not as easy as people think," defending champion Webb Simpson said after a 75 put him six shots behind the clubhouse lead. "I heard 15, 16 under floating around. And it's going to be a normal U.S. Open winning score, I think."

Horschel hit all 18 greens in regulation, a stellar achievement at a regular tour event, let alone the U.S. Open. It sent USGA officials searching for hours to find the last time anyone failed to miss a green in the toughest test in golf. Records of that detail only go back as far as 1989. That last documentation of someone doing that was Johnny Miller when he closed with a 63 at Oakmont to win in 1973.

David Graham used his putter on every hole -- three from the fringe -- when he shot 67 to win the 1981 U.S. Open at Merion.

"I didn't know I hit every green until I walked off 18," Horschel said. "It's a cool thing. But like I said, it's not the first time I've hit all 18 greens. I've done it plenty of times in my career. Obviously, it's at a U.S. Open, but I think the softness of the greens helped that."

Pan played nine holes and was even par, along with Ian Poulter, who was plodding along in plaid at 1 under for his round through 14 holes. John Senden of Australia had a 71 and Nicolas Colsaerts of Belgium shot 72 to finish at 1-over 141.

Mickelson, equipped with a full night of rest after his cross-country trip Wednesday from his daughter's eighth-grade graduation in San Diego, began with a three-putt bogey and appeared ready to pull away with a shot that nearly spun back into the hole at No. 8. He missed the birdie putt from 4 feet. Then he hit a beautiful tee shot over the water to a dangerous front pin on the par-3 ninth to about 7 feet. He missed that one, too.

Lefty three-putted from 20 feet on No. 12, and then flew a wedge over the green into a plugged lie for bogey on the par-3 13th. He kept battling until ending on a sweet note. With that birdie putt on his final hole, Mickelson was under par through 36 holes for the seventh time in the U.S. Open. The previous six times, he was a threat to win on Sunday. Mickelson has five silver medals as a runner-up, and all he wants is another chance.

"I just like being in the mix," he said. "I think it's fun having a chance heading into the weekend. The way I have control off the tee and as good as the putter is -- even though it didn't show today -- I'm very excited about the opportunity this weekend."

Horschel doesn't lack for confidence, even though the 26-year-old from Florida won for the first time on the PGA Tour just two months ago in New Orleans. He is an explosive player, capable of running off birdies without notice. For this championship that meant keeping the ball in play.

His only bogey was on the 13th hole, the short par 3 and the easiest at Merion.

"I was not in the zone, trust me," Horschel said. "This golf course, even though it's soft, is still a tough golf course. I know what 'in the zone' is for me. I don't get nervous, I just see the shot and go. And I saw the shot and I went with it, but I was still nervous with a lot of them. Your misses here can be bad if you miss in the wrong spot."

Graham's great round won him the U.S. Open. Horschel still has a long way to go. Considering this packed leaderboard, it feels as though the tournament hasn't even started.

For all his travails, Woods was only four shots behind. So was Rory McIlroy, who also had a 70.

"It tests every aspect of your game," McIlroy said. "There were people talking about 62s and 63s at the start of the week and, I mean, I never saw that at all. I still think that something very little under par is going to win this week. If or if not that, around even par."

Indeed, the real winner so far has been Merion.

For such a short course -- it measured 6,901 yards from tee to wicker basket -- this century-old track had everyone's attention.

"You were convinced it was going to be scoring records and 62s and obnoxious scoring," Ogilvy said. "Did one player say that? Not many, anyway. The players said it was pretty hard, didn't they? Today was hard."

Everything looked like a grind for Woods, who said he first hurt his left arm at The Players Championship -- he didn't say where or how -- when he won at the TPC Sawgrass a month ago. He dangled the arm and occasionally grimaced with shots out of the rough on No. 12, No. 4 and No. 8.

He was more interested in his game, and that didn't cause him much pain at all. And even though he was halfway through his quest to end five years without a major, Woods was keeping his head down.

"Just keep grinding," he said. "You just don't ever know what the winning score is going to be. You don't know if the guys are going to come back. We have a long way to go, and these conditions aren't going to get any easier."